Reagent test strips are widely used in the field of clinical chemistry. A test strip usually has one or more test areas, and each test area is capable of undergoing a color change in response to contact with a liquid specimen. The liquid specimen usually contains one or more constituents or properties of interest. The presence and concentrations of these constituents of interest in the specimen are determinable by an analysis of the color changes undergone by the test strip. Usually, this analysis involves a color comparison between the test area or test pad and a color standard or scale. In this way, reagent test strips assist physicians in diagnosing the existence of diseases and other health problems.
Color comparisons made with the naked eye can lead to imprecise measurement. Today, strip reading instruments exist that employ reflectance photometry for reading test strip color changes. These instruments accurately determine the color change of a test strip within a limited wavelength range or bandwidth but sometimes fail to measure minute color inconsistencies outside the limited bandwidth. For example, such an instrument can fail to detect traces of blood within a urine specimen on a MULTISTIX.RTM. reagent strip of the type sold by Miles Inc., Diagnostics Division, of Elkhart, Ind. 46515. After the urine specimen contacts the test pad of a MULTISTIX.RTM. reagent strip, intact blood cells appear as tiny green blotches on the yellow test area. Existing strip readers detect the overall color of the test pad but can ignore the small blotches of green. In addition, existing strip reading instruments using one or more wavelengths can lead to a false positive bilirubin result in the presence of interference from indoxyl sulfate. Visually, the atypical color is easily detected but not by prior strip readers that only analyze a limited bandwidth of the entire visual spectrum.
The first commercially available strip reading instruments of this type were effective but unable to adequately cope with the large numbers of specimens handled by clinical laboratories. These instruments require inserting a single test strip, reading the test strip and removing the test strip from the instrument before the instrument can analyze the next test strip. Moreover, with certain instruments the speed of operation is limited by the requirement of precise placement of the strip in the instrument.
Automation of strip reading instruments has significantly improved the speed with which specimens are processed. U.S. Pat. No. 5,055,261 discloses a multiple-strip reading instrument utilizing reflectance photometry to read test strips. An operator sequentially places the test strips transversely in a loading area. A blotter arm orients the test strips on rails extending from the loading area to one or more reading stations employing read heads and then to a waste receptacle.
This instrument provides for the reading of reagent strips with multiple test areas having varying incubation times. An indexing mechanism in timed relation with the blotter arm incrementally advances the strips in spaced parallel relation a predetermined distance along the rails. After each incremental advance, each test strip dwells for a predetermined time period in its new position. Consequently, individual test strips sequentially advance to a reading position where, during the dwell period, certain test areas are read. Subsequently, the instrument advances the test strip to the next reading position where the instrument reads the other test areas on the test strip with longer incubation times. This arrangement is somewhat inflexible to variations in incubation times for varying test strips because the timing of this instrument accounts for the distance that the test strip travels from the loading area to the read heads, the incubation times for a certain test area and the indexing rate of the indexing mechanism. Thus, if the instrument reads a test strip having test areas with different incubation times than the typical test strip, the instrument can obtain inaccurate results.
The instrument embodied in U.S. Pat. No. 5,143,694 also transports test strips at a right angle to their longitudinal direction from a strip loading area, along a transport path under the read heads to a waste receptacle. In order to obtain accurate results, these instruments require that the test strips be read at the appropriate time.
A common feature of these instruments is a visual and auditory prompt signalling the operator to dip a strip in the sample and place it in the loading area of the instrument. Typically, these prompts occur at fixed time intervals, such as every 10 or 12 seconds. Unfortunately, operators frequently fail to comply with the prompts by either not understanding or choosing to ignore the manufacturer's instruction about immersing the test strip when the tone is presented. This timing differential between the instrument prompt and when the operator actually dips the test strip can cause a degradation of measurement results. Moreover, forcing the user to dip and place strips as dictated by the instrument adds pressure onto the operator and can lead to human error.